The need to share a host telephone system between a central office line and an additional system line has existed for some time. This need is most prevalent when the host telephone system is used with a limited number of central office line ports or when in a residential environment being served by a single central office line. Sharing the central office line interface with the additional system avoids the increased costs associated with extra central office trunk interfaces or dedicated additional systems. The additional systems can be, for example, paging/intercom circuits having a paging/intercom line pair. However, while the description is given with respect to a paging/intercom additional circuit interfacing with the central office via a door phone controller, it is understood by those skilled in the art that other types of additional circuits may also be interfaced.
When using the host telephone system with a additional system, e.g. an electronic key telephone system (EKTS), the features of the additional system, such as paging/intercom, can be accessed in different manners. Typically, for a paging circuit using a dedicated line, the paging feature can be accessed via a central office trunk port or, in an EKTS system, via a page port. However, the page port is usually limited in performance. This is because the port provides only a one-way page audio path such that dual-tone multi-frequency (DTMF) (used for programming or zone paging) or talk-back (talk-back being audio generated at the speaker) is not passed. If either of these features, i.e. DTMF or talk-back is required, then the user must dedicate a central office trunk port for the exclusive purpose of performing paging.
One of the first attempts at sharing the interface allowed the host telephone to access either the paging system or the central office line by pressing a predetermined DTMF digit on the telephone keypad. If a central office telephone call arrived while the paging system was in use by the user, then a "call waiting tone" was returned to the host telephone. If a central office telephone call arrived while the host telephone was on-hook, then the call is passed through as if the paging device was not present. However, while this operation satisfied the requirements of sharing the central office line with a paging/intercom system, several disadvantages with respect to the user arise. For example, once the host telephone initially goes off-hook, the user receives a non-standard dial tone. After receiving this non-standard dial tone, the user must dial a certain digit, e.g. usually "9", to receive the central office dial tone. Alternatively, the user could wait a pre-programmed amount of time after going off-hook before receiving the central office dial tone.
An alternative operation for sharing the central office line with an additional system avoided some of the above-described disadvantages by immediately providing the user with a central office dial tone upon going off-hook. However, this solution created a further problem in that if the user desired access to the paging circuits when the telephone is connected to the central office line, the usual signals for switching between the central office line and additional systems, e.g. "hook flash" signals and all DTMF digits, would initially be sent to the central office switch. Therefore, the central office switch would function in accordance with the features available to it. This has the disadvantage of making the access to the paging circuits dependent upon the central office switch rather than the door phone controller.
There is therefore needed an interface provided in a door phone controller which allows the user to access the additional circuits upon taking the host telephone system off-hook.